Physical Sciences Division Research Highlights

Putting the Pressure on Carbon Dioxide

At a certain pressure, a carbon dioxide film that forms on kaolinite releases, allowing fewer interactions to occur between the carbon dioxide and the mineral. Artwork from this research graces the cover of Environmental Science & Technology Letters. Copyright 2014. American Chemical Society. Enlarge image

Results: In a
surprising turn, carbon dioxide goes from quickly bonding with to jumping off the
surface of the layered clay kaolinite when the pressure increases beyond a
critical value, according to scientists at Pacific Northwest National
Laboratory (PNNL). When the CO2 pressure is sufficient to raise its
density to 0.40 grams per cubic centimeter, CO2 becomes more mobile
and leaves, or desorbs, from the kaolinite surface. Below this pressure, CO2
readily interacts with the clay, as noted in other studies. Here, the challenge
was determining why the CO2's behavior was so peculiar when its
density reached this level. By bringing together experiments and computational
simulations, the team discovered that it was more energetically favorable for
CO2 to move into the liquid-like supercritical phase as opposed to
being bonded as a film on the clay surface as the pressure increases. The
research team's findings were published in the February issue of Environmental Science & Technology
Letters and were highlighted on the journal cover.

"This is a fundamental molecular-scale study," said Dr. Vassiliki-Alexandra
Glezakou, a PNNL computational chemist on the study. "But, it provides vital
information for reservoir engineers seeking to extract methane and permanently
store CO2 in shales."

Why It Matters: Carbon dioxide's behavior with clay minerals vitally impacts carbon
sequestration, essentially trapping combustion-sourced pollution and then
injecting it underground as a supercritical fluid, which is a gas that behaves
like a liquid. PNNL is investigating
methods where CO2 can be used to discharge natural gas or methane
from fractured shale, providing more domestically sourced energy to heat homes
and drive industry. The shale prefers CO2 to methane, so it releases
the methane and bonds to the CO2.

"Our results are
key building blocks to eventually predicting the optimum conditions for
storing carbon dioxide in depleted shale gas formations while extracting
additional natural gas," said Todd Schaef,
a PNNL geochemist on the study. "In the shale reservoir, we need to know the
best conditions for carbon dioxide to attach to minerals like kaolinite. If you
overpressurize, the carbon dioxide won't attach."

Methods:
The researchers began with
kaolinite, an abundant clay mineral. In this study, kaolinite is a model mineral
system, allowing the scientists to focus on specific properties. "Even though
we are studying just one mineral, the findings can be transferrable as there
are similarities between many minerals," said Glezakou.

Kaolinite
primarily absorbs CO2 on exposed surfaces. Scientists have long
reported a linear adsorption dependence with pressure. Up to a certain point.
Then, the adsorption reverses, regardless of increasing pressure.

The team studied the
interactions using a quartz crystal microbalance, which measured the amount of CO2
sorbed onto the mineral surface. When they could not quite distinguish the
mechanisms involved, they simulated the interactions with density functional
theory (DFT) calculations. The experiments and simulations were done using resources
at the U.S. Department of Energy's (DOE) EMSL. "Without EMSL's computational
resources, we could not have done these simulations," said Glezakou.

"Carbon
dioxide is a well-studied molecule that still has secrets," said Schaef.
"We and many others have been working
with carbon dioxide and various mineral systems for years; yet nobody had
explained its odd adsorption behavior until we asked why."

What's
Next? More studies on CO2's behavior
with different minerals, such as swelling clays, which are another component of
the shale, are next. The scientists will determine if the behavior seen on
kaolinite occurs on other materials. "We also need to begin the process of
incorporating the findings from the molecular-scale simulations into our
reservoir-scale simulation models so we can test new ideas for injecting CO2
to enhance methane recovery from depleted shale gas formations," said Dr. Pete
McGrail, a PNNL Laboratory Fellow who also participated in the study.